1. Field of the Invention
This invention relates to a preheating system with an incipient calciner suitable for preheating and incipient calcination of raw materials of cement, alumina, lime stone and the like, and more particularly to a dust preheating system with an incipient calcination furnace which can realize improvements of performance quality in both the combustion of a fuel and incipient calcination of the dust of a raw material in the incipient calcination furnace.
2. Description of the Prior Art
FIG. 1 shows a flowchart of an exemplary dust burning system for raw materials of cement, in which arrows of solid line indicate a flow direction of gases and arrows of broken line indicate a flow direction of raw material dust.
This system is mainly composed of a preheater/incipient calciner 1 consisting of dust separators C1 to C4 in the form of cyclones or the like and an incipient calcination furnace 2, a main combustion furnace 3 in the form of a rotary kiln or the like, and a clinker cooler 4. The powdery raw material which is fed through a chute 5 successively flow down through the first to third stage cyclones C1 to C3, while hot exhaust gases from the combustion furnace 3 and the incipient calcination furnace 2 are sucked by an induced draft fan 8, flowing up through the preheater 1. Therefore, heat exchange between the powdery raw material and hot gas is repeated in the duct 7 and cyclones C1 to C3. The preheated powdery raw material is fed to the incipient calcination furnace 2 through a chute 14 from cyclone C3 of the second lowest stage of the preheater 1.
On the other hand, combustion takes place in the incipient calcination furnace 2 which receives hot secondary combustion air from the clinker cooler 4 through a combustion air duct 13 in addition to the supply of a fuel and primary combustion air from a burner 6a. By the heat of this combustion and of the exhaust gases from the combustion furnace 3, the powdery raw material which is charged through the chute 14 is preliminarily calcined. The powdery raw material which has undergone the incipient calcination through the incipient calcination furance 2 is fed to the lowermost cyclone C4 along with the combustion exhaust gas, where the dust is separated from the combustion gas and sent to the combustion furnace 3 through the chute 15. The powdery material is subjected to a necessary heat treatment in the combustion furnace 3 and formed into clinker by the heat resulting from combustion of a fuel which is supplied by a burner 6b located at the end of the furnace 3, discharging the clinker to the cooler 4 for cooling.
The clinker cooling air is supplied by a forced draft fan 10 and part of hot air resulting from heat exchange with the clinker is circulated to the incipient calcination furnace 2 and combustion furnace 3, discharging excess air by the inducing draft fan 9. The clinker which is discharged from the clinker cooler 4 is transferred to a next processing stage by a conveying means 11.
FIG. 2 is a schematic illustration showing details of the preheater arrangement in the vicinity of the incipient calcination furnace, which is employed for the explanation of the construction and functions of the incipient calcination furnace.
Namely, in the particular example shown, the incipient calcination furnace 2 is the form of an upright cylinder, which is provided with a combustion chamber 2a and a mixing chamber 2b on the lower and upper sides of a constricted orifice portion 2c. The lower end of the combustion chamber 2a is formed in an inverted truncated-cone shape with its sectional area gradually reduced in the downward direction, terminating in an opening 2d which is connected to the combustion furnace 3 through an end cover 12. A combustion air duct 13 which guides the combustion air from the clinker cooler 4 is radially or tangentially connected to an inlet port 2e provided in a lower portion of the side wall of the combustion chamber 2a, and a burner 6a which supplies a fuel is embedded in a position proximate to the junction of the ceiling wall of the combustion air duct 13 and the side wall of the combustion chamber 2a of the incipient calcination furnace 2, directing the burner 6a toward the hot combustion air which is drawn into the combustion chamber 2a. Further, a chute 14 for the preheated material from the cyclone C3 in the second lowest stage of the preheater 1 is connected to a position above the burner 6a, and directed toward a combustion zone 16 which is formed in the combustion chamber 2a by the fuel supplied from the burner 6a. On the other hand, a combustion gas outlet 2f of the mixing chamber 2b is connected to the cyclone C4 in the final stage of the preheater 1.
In operation, the preheated material dust from the cyclone C3, the second one from the lowest, of the preheater 1 is fed into the combustion chamber 2a of the incipient calcination furnace 2 through the chute 14, and mixed and stirred in the combustion chamber 2a by ascending exhaust gas from the combustion furnace 3, forming fluidized gas streams. The combustion air which is drawn from the clinker cooler 4 is introduced into the fluidized gas streams through the combustion air duct 13, while a fuel is supplied from the burner 6a above the air supply port 2e through which the combustion air duct 13 is opened into the combustion chamber 2a, thereby effecting combustion in the fluidized gas streams.
Accordingly, the powdery raw material which is fed into the combustion chamber 2a through the preheated dust chute 14 undergoes reactions of incipient calcination by absorption of the heat resulting from combustion of the fuel and the sensible heat of the exhaust gas from the combustion furnace 3, passing through the constricted orifice portion 2c along with the combustion gas, and then admitted into the mixing chamber 2b. After completely burning combustible components of the combustion gas in the mixing chamber 2b, the material is discharged into the cyclone C4 in the lowermost stage of the preheater 1 through the opening 2f.
For burning the fuel in the incipient calcination chamber in the above-described manner, the burner 6a is mounted in such a manner as to be directed toward the hot air flowing into the combustion chamber 2a, for effecting the combustion in as good a condition as possible.
In a case where the preheating raw material is thrown toward the combustion zone 16 in the combustion chamber 2a from the second lowest cyclone C3 of the preheating section 1 which supplies the preheating raw material to the incipient calcination furnace, as shown in FIGS. 1 and 2, there is an advantage in that the reactions of incipient calcination can be accelerated since the powdery raw material is promptly heated to a high temperature in the combustion zone. However, such increases the concentration of the powdery raw material in the combustion zone 16, consequently lowering the combustion temperature in the combustion zone 16 and resulting in an unsatisfactory quality of combustion.
On the other hand, in a case where the preheating dust chute which supplies the incipient calcination furnace 2 with the preheating raw material from the second lowest cyclone C3 of the preheating section 1 is connected at a position distant from the combustion zone 16 in the combustion chamber 2a to the circumferential direction in cross section of the incipient calcination furnace 2, namely, to a position 14' indicated by broken line in FIG. 2, the concentration of the powdery raw material in the combustion zone 16 becomes relatively lean and the quality of combustion of the fuel is improved by a temperature elevation in the combustion zone 16. However, since the heating of the powdery raw material in the combustion chamber 2a becomes slower, the incipient calcination reactions proceed at a lower velocity, resulting in an inferior incipient calcination quality and production of an increased amount of NOx (nitrogen oxides) due to the temperature elevation in the combustion zone 16.
Under these circumstances, the present inventor previously proposed a dust preheating system with an incipient calcination furnace in which, as disclosed in Japanese Patent Application No. 55-105643 (see Laid-Open Patent Application No. 57-34054), the preheating material to be fed to the incipient calcination furnace is divided into two parts, one part being fed to the combustion zone and the other part being directed away from the combustion zone and toward the exhaust gas which flows into the incipient calcination furnace from the combustion furnace, thereby adjusting the temperature of the combustion atmosphere for improving the incipient calcination quality and suppressing the production of NOx while maintaining satisfactory combustion quality. In this previously proposed system, the powdery raw material undergoes the incipient calcination reactions to a sufficient degree with regard to the part which is fed to the combustion zone, but not the other part which is fed to a region remote from the combustion zone. Thus, such still needs improvements in overall incipient calcination quality.